1. Field of the Invention
The present invention relates to image reading apparatuses, such as an image scanner, a facsimile machine and a copying machine, and more particularly to an image reading apparatus that uses a close-contact type image sensor.
2. Description of Related Art
Heretofore, there has been known an image reading apparatus for reading an image on an original to produce a digital signal for each pixel. More specifically, the image reading apparatus is provided with a plurality of image reading sensors each having a one-dimensional array of pixels, and the plurality of image reading sensors are arranged in a row in the direction of the array of pixels so as to cover the required image reading range and are configured to read an image on an original while moving in the direction perpendicular to the direction of the array of pixels, thereby producing a two-dimensional digital signal.
However, in such a type of image reading apparatus, as the reading resolution becomes higher, it becomes difficult to accurately set the distance in a joint portion between each pair of adjacent image reading sensors of the plurality of image reading sensors to a distance approximately equivalent to one pixel. If the distance in a joint portion between adjacent image reading sensors is large, when an image on an original having periodicity, such as a halftone-dotted image in print, is read, missing data occurs at a part of image data read corresponding to the position of the joint portion between adjacent image reading sensors, so that streak-like noises would be unfavorably generated, thereby causing deterioration of image quality.
In order to compensate for such missing data in the joint portion between adjacent image reading sensors through interpolation, for example, in Japanese Laid-Open Patent Application No. 2003-8853, there is proposed a method of performing one-dimensional filtering computation. In the proposed method, such filtering computation is performed at all elements of image data whether in interpolation positions (positions of missing data) or not. Then, a difference between a result of filtering computation and actual data (luminance value) in an arbitrary data position other than any interpolation position is obtained, and the result of filtering computation that has been found to be nearest the actual data is selected as a result of filtering computation in an interpolation position.
However, in the above-mentioned proposed method, since a comparison is made between each of a plurality of results of filtering computation and the associated actual luminance value in all pixels except joint portions of image reading sensors, a complicated processing operation is required. Also, in order to perform high-precision interpolation, it is necessary to increase the number of pieces of reference data. Therefore, the above-mentioned proposed method has limitations from the viewpoint of simply and precisely compensating for missing data in joint portions of image reading sensors through interpolation so as to obtain a high-quality image.